1. Field of the Invention
The present invention relates to a line-illuminating device having a bar-shaped light guide and a light-emitting unit and to an image-scanning device in which the line-illuminating device is incorporated.
2. Description of the Background Art
A contact-type image sensor (i.e., an image-scanning device) is used as a device for scanning a document using a facsimile machine, a copying machine, an image scanner and the like. The contact-type image sensor is provided with a line-illuminating device for linearly illuminating a document surface along a main scanning field.
A line-illuminating device using a light guide is known. For example, Patent Document 1 proposes a line-illuminating device in which a light-emitting unit is provided on one end of the light guide and a light-scattering pattern is formed to diffuse and reflect the light from the light-emitting unit along the longitudinal direction of the light guide. A plurality of light sources (LEDs) of the light-emitting unit is provided on a normal line of the light-scattering pattern and a light-emitting surface is formed in a direction differing from the normal line.
Disclosed in Patent Documents 2 and 3 is the type of line-illuminating device in which light-emitting units are provided on both ends of the bar-shaped light guide or part of the surface of the bar-shaped light guide is provided to serve as the light-scattering surface.
[Patent Document 1] Japanese Patent No. 3083092
[Patent Document 2] Japanese Unexamined Patent Publication No. HEI 6-148435
[Patent Document 3] Japanese Unexamined Patent Publication No. HEI 7-14414
FIG. 7 is a view explaining a coupling section of a light-emitting unit and a bar-shaped light guide used in the above-mentioned Document 1. In the conventional light-emitting unit, a substrate 100 forming a circuit is provided with a window 101 for coupling. Each light-emitting element 102, 103 and 104 of three primary colors of red, green and blue is disposed within the window 101. Electricity is fed to the circuit for light emission formed in the substrate 100 through lead wires 105.
Each light-emitting element has an area about 0.3 mm square and a space of 0.8 mm or more must be provided between the light-emitting elements to avoid overheating due to heat dissipation. As a result, the window 101 has a size of about 1.1 mm×2.8 mm. If the window 101 is larger than a cross section of a light guide 106 when installed, the light comes through. Thus, the cross section of the light guide 106 must have such a size as to completely cover the window 101.
In the known system, since the cross-sectional shape of the bar-shaped light guide is designed to completely cover the window 101, limitations are imposed on the size of the cross section of the bar-shaped light guide. As a result, it is difficult to set the density of the energy of light traveling inside the bar-shaped light guide above a certain level. When the density of the light energy is low, it is not possible to effect radiation with such directional characteristics as to brightly illuminate a narrow area of a document surface.